Assessment of FRP-confined concrete : understanding behavior and issues in nondestructive evaluation using radar

Abstract

Increase in the use of fiber-reinforced polymer (FRP) composite materials for strengthening and retrofitting of concrete columns and bridge piers has urged the development of' an effective non-destructive evaluation (NDE) methodology. Radar technologies have shown great potential for assessing the structural and material integrity of FRP-confined concrete systems. In developing such technology, an interdisciplinary approach must be pursued by integrating contributions of various engineering fields. Under this framework, this thesis aims at establishing fundamental knowledge in two particular research areas: the mechanics and damage behavior of FRP-confined concrete and the characterization of electromagnetic (EM) properties for concrete and FRP materials. Research on mechanics and damage behavior of FRP-confined concrete involved a thorough literature survey on the state-of-the-art understanding of the subject and the execution of an experimental program for load-deformation characterization of FRP-confined concrete cylindrical specimens subjected to monotonic axial compressive loads. Based on the experimental results and comparative studies with recent analytical models, the experimental program was extended to the characterization of specific damage levels using a volumetric deformation metric. Visual inspection of the concrete core of specimens previously loaded to levels close to ultimate failure revealed the existence of significant residual volumetric strains, which upon the removal of the FRP jacket could provoke severe concrete cracking and catastrophic failure.

(cont.) Research on dielectric property characterization of concrete and FRP materials led to the development of an integrated methodology for estimating the complex permittivity of low-loss materials in general using free-space measurements of EM wave transmission. Such development required theoretical modeling of EM wave propagation through dielectric media and experimental measurements of transmission coefficients. Validation studies were performed using Teflon, Lexan, and Bakelite materials whose dielectric properties are established in literature. The methodology was then applied to concrete and FRP materials. Establishing minimum criteria for specimen dimensions and optimal frequency bandwidths is still required before the proposed methodology can be used in field applications. Additionally, exploratory research on the assessment of FRP-confined concrete using radar technologies was conducted. Preliminary results indicate potential of such technologies for detecting features related to the presence of rebar, air cavities, delaminations, and mechanical damage in FRP-confined concrete columns.